The claim behind "theory of mind" is that certain core conceptions organize and enable our everyday understanding of the social world. In particular, social cognition is based on thinking of people in terms of their mental states--their beliefs, desires, hopes, goals, and inner feelings. This everyday assumption of mind is powerful and constraining. It leads us to try to use the mind and increase its powers, to share inner experiences, to distinguish between purely imaginary and real events, and to interact with other persons by searching for and reaching out to their underlying mentalities. A mentalistic construal of persons is fundamental not only to adults; some essential parts of it develop early in childhood. This raises intriguing questions: (1) When do children know what about basic mental-psychological states--beliefs, desires, emotions? (2) When and how do mental state understandings cohere into what sort of larger naive psychology? (3) When do children's mentalistic understandings become central to their everyday lives? (4) How are these conceptions developed and socio-culturally transmitted; what factors shape understanding; to what extent is a mentalistic construal of persons widespread across cultures vs. limited to our society? I propose 12 interrelated investigations to address these questions. The investigations include meta-analytic, laboratory, and conversational studies that build on and advance my recent related research. The studies focus especially on: children's explanations for and coherent mental state understandings of persons; the processes of change, including parent-child interchanges, that shape children's developing conception; the preschool years--a period of crucial change and transition for "theory of mind" understandings--with the inclusion of 2-year- olds who bridge the gap from infancy to early childhood.